Shutting out the voters might seem like a sensible strategy, but it can only lead to one result: a slap in the face come polling day.

Terrified by the fear of another Gillian Duffy moment, the parties have adopted a cautious approach to these precious few weeks that tries to avoid calamity at the expense of giving themselves a chance of actually changing the game. Elections should not be shutting-down operations, but that is the approach taken by the Conservatives and Labour.

Lynton Crosby might be placing his faith in a late incumbency swing, but there are no signs of it materialising yet. Labour stands more to gain from seeking a sudden moment where longstanding perceptions shift. As #milifandom shows, any progress they have made in this campaign isn't really of their making.

It's ironic, really. Gordon Brown's 'bigotgate' disaster helped bring down the New Labour government because by writing off Duffy, Brown was writing off millions of other voters too. Yet in strenuously trying to avoid a similar fate another mass insult is taking place: voters are being removed from the political process of the main air-war campaigns altogether.

It leaves the media left with nothing better to do than wonder what a hung parliament means. It explains the focus on the SNP, a party that can't expect to take more than four per cent of the national vote share but could end up seizing 50-odd seats – and advancing its long-term agenda of breaking up Britain. For this is a campaign overshadowed by the far more important contest of 2014, when the future of this country was at stake. That matters a lot more than who governs the UK over the next five years. But the Conservatives are prioritising power over the preservation of the union. It is ugly politics, made no easier to accept by the fact that a preoccupation with the SNP's role is exactly what the Tories hope to achieve by shutting down genuine debate elsewhere.

What this campaign shows is an abdication by both of the main two parties of any sense they can restore the old politics that served them so well in the 20th century. Those steady rotations of power worked well for Labour and the Conservatives for many decades. But their two-party system led to over-centralisation and an obsession with consistency that belied the reality of both parties. They were always broad churches characterised by struggles for influence between competing sub-parties. Now, with the ideological tensions of the 1980s subdued, those fringes have begun breaking off. Despite the desperate need for the Tories and Labour to find a way of adapting, of combating this drift to the edges that has amplified extreme voices, the 2015 campaign has shown neither really has an answer.

But adapt they must. In the digital age democracy must be about more than targeted emails and ironic hashtags. In the next parliament whichever party is in power must find ways of embracing the new expectations of voters about what British politics means. Yes, in a hung parliament that means working harder to secure consensus in the Commons. But it's about more than that – it's about persuading the general public that they aren't nearly as shut out from everyday politics as they are from the stage-managed bore-fests of this interminable campaign.

For years the idea of Ed Miliband in No 10 was judged unthinkable, even laughable. But with the general election just over a fortnight away, a consensus is gradually emerging that his premiership is now the most likely outcome.

Winning general elections is all about managing expectations – so it really matters that many are quietly deciding the Labour leader is more likely to end up in Downing Street after polling day.

It all comes down to numbers. Try playing the BBC's majority-builder game, which offers some likely scenarios and gives you the chance to work out plausible governments. More often than not you're likely to find the Conservatives fall short, even with Lib Dem and DUP support. Labour is often relying on either the SNP and/or the Lib Dems to get into power.

That reflects the balance of probabilities as assessed by the Political Studies Association, a group of unspeakably clever academics who suggest the most likely scenario is bad news for Cameron. "The single most likely outcome is at the bottom of the pie chart," says Dr Stephen Fisher of the University of Oxford. "That is a seriously hung parliament with the Conservatives as clearly the largest party but a majority on the left, including the SNP and Liberal Democrats."

His findings are supported by the academics at electionforecast.co.uk, which give the Conservatives a mean 284 seats, compared to 276 for Labour, 25 for the Liberal Democrats and 41 for the SNP.

Exactly how this will all play out remains to be seen. There are lots of variables, but the overall assessment is one that the New Statesman's spin-off website May2015.com makes rather boldly. "Who will win the 2015 general election? With only 18 days to go, we now think it is likely that Ed Miliband will become prime minister this summer," it predicts. The maths just works better for Labour, it seems, because "there are few scenarios in which David Cameron will survive a vote of confidence".

The pollsters are starting to agree. YouGov's Peter Kellner, writing today, says he thinks the Tories are losing both the air and ground war of the campaign, which has wiped out their previous, slim advantage. He expects a late incumbency bonus will help Cameron's cause, but that won't be enough to escape the logic that everyone seems to be coming to.

"Even if the Conservatives do remain the largest party, a result anything like my forecast would probably lead to Miliband becoming prime minister," he argues. "Even if he could secure the support of the Lib Dems and around ten Ulster Unionists, Cameron would be able to count on 317 MPs, while Labour, the SNP and the smaller left-of-centre parties in England, Wales and Northern Ireland would have around 325." On those figures, it's clear the Tories couldn't govern. But Miliband would need the support of the SNP.

Kellner's assessment highlights just how tight this is going to be. Governing is possible with limited majorities, but it is not straightforward. Votes can and may very well be lost. And there are other uncertainties, too – like how the party's supporters will respond to any particular deal.

This is where new polling from Survation out today comes in handy. Of the 634 voters it spoke to who ranked Labour as their first preference, 49% said they would prefer a minority government, compared to 30% who wanted to see a coalition with the Liberal Democrats. That suggests a division which is likely to leave no-one especially happy.

The same is not true for Conservative supporters, interestingly. While 42% would like to see a Tory minority, 41% would prefer a repeat of the last five years. Experience of sharing power seems to have accustomed right-wingers to the idea. Their real problem, as many are now concluding, is that the odds are stacked against them.

That raises questions of legitimacy. If the Conservatives are the largest party in the Commons but can't command a majority, some fear they're not going to exit power quietly. "They would instantly denounce such a government as illegitimate," Jonathan Freedland of the Guardian writes. "Backed by a Tory press in full cry, they would say Miliband had no mandate. They would call him a squatter in Downing Street, insisting he had usurped power… cartoons would appear of Red Ed in the silver medal position on an Olympic podium trying to wrench the gold from David Cameron's grasp.

The struggle for legitimacy is likely to occur in most scenarios as the real weaponisation of 2015 takes place. Forget the NHS; it's our constitution which could end up being commandeered by the losing side after polling day.

There have been the usual gripes about factual inaccuracies and scorn for candidates' tactical voting claims, but nothing has quite topped the SNP's suggestion that a Lib Dem MP is retiring – when he very much isn't.

But then the stakes are rather high north of the border, aren't they? Maybe this explains why Labour seems to be working quite so desperately hard to demonstrate they know which part of the country they're in.

For some reason it does appear rather as if it's Labour's leaflets which are getting the most stick. They don't have a monopoly on candidates which aren't particularly photogenic, it's true, but still….

This parliament has had 48 months since first gathering in mid-May 2010 to sort out the arrangements for the next coalition. So why is it that the MP tasked with overseeing our constitutional arrangements is now demanding a change of plan?